Sciatica - What Causes it?

What Causes it?

The sciatic nerve is the largest nerve in the body. It starts in the low back at lumbar segment 3 (L3). The nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine.

The nerve is named for the upper vertebral body that it runs between (for example, the nerve that exits at L4-L5 is named L4). The nerve passing to the next level runs over a weak spot in the disc space, which is the reason discs tend to herniate (extrude) right under the nerve root and can cause leg pain.

The sciatica symptoms (pain, numbness, tingling, weakness) are different depending on where the pressure on the nerve occurs. For example, a lumbar segment 5 (L5) nerve impingement can cause weakness in extension of the big toe and potentially in the ankle (foot drop) (See Diagram).

Disc herniation and disc degeneration due to aging are the most common causes of low back pain. Other problems can also cause this pain, however.
Lumbar Degenerative Disc Disease. Over the years, the disc can degenerate and produce low-grade inflammation and irritation. This age-related condition is the major source of chronic low back pain.
Herniated Disc. A herniated disc, sometimes, but incorrectly, called a slipped disc, is widely held to be the most common cause of severe back pain and sciatica. A disc in the lumbar area becomes herniated when it ruptures or thins out and degenerates to the point that the gelatin within the disc protrudes outward.
It is commonly believed that that low back pain most often occurs if this material extrudes (that is, it balloons into the area outside the vertebrae or breaks off from the disc) far enough out to press against the nerve root, most often the sciatic nerve. Recently, however, researchers are finding that the presence of such a pinched nerve does not necessarily relate to the severity of the pain. In fact, as people age, disc bulging and protrusion are very common occurrences, and in most cases do not cause any back pain. And, sciatica pain is sometimes present when there is no bulging or extruding of the discs. Experts increasingly believe, then, that low back pain associated with disc abnormalities may result from factors other then compressed nerves.
The Annular Ring. Increasingly, research is focusing on tears in the annular ring, which is the fibrous band that surrounds and protects the disc. The annular ring contains a dense nerve network and high levels of peptides that heighten perception of pain:

  • Tears in the annular ring are a frequent finding in patients with degenerative disk disease.
  • Some cases of chronic low back pain may be caused by inward growth of nerve fibers into the annular ring, which trigger pain within the intervertebral.
Muscle and Ligament Injuries Other than age-related degenerative disk disorders injuries in the muscles and ligaments supporting the back are the major causes of low back pain. Spinal Stenosis Spinal stenosis is the narrowing of the spinal canal. This typically develops as a person ages and the discs become drier and start to shrink. At some point in this process, any disruption, such as a minor injury that results in disc inflammation, can cause impingement on the nerve root and trigger pain. pain from spinal stenosis can occur in both legs or can cause sciatic pain. Spinal stenosis occurs mostly in the elderly with degenerative osteoarthritis, but it can sometimes be caused by other problems, including infection and birth defects. Miscellaneous Abnormalities A number of conditions that affect the joints, vertebrae, or nerve roots can cause back pain:
  • Spondylolisthesis is a condition in which one vertebra has slipped forward over the other. This is also a cause of sciatica.
  • The facet joints can wear down. In such cases, pain occurs on arching the back or when walking.
  • In some cases a segment (consisting of two vertebra and their common joint and disc) becomes unstable when its parts wear down.
  • Some patients may have scar tissue that traps the nerve roots in the lower spine and causes sciatica.
piriformis Syndrome Some experts believe that one cause of sciatica pain is the entrapment of the sciatic nerve deep in the buttock by the piriformis muscle. It usually develops after an injury. Others believe there is no real evidence that this condition, known as piriformis syndrome, causes any sciatic pain.
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